1. Field of the Invention
The present invention relates to a novel process for producing an aluminum oxide dispersion throughout a silicon nitride matrix, generally believed to be a solid solution, the process including obtaining reactants from readily available sources of raw material.
SIALON is defined herein to mean an intimate dispersion of aluminum oxide throughout a silicon nitride matrix. It is generally believed that upon sintering the material becomes a solid solution of aluminum oxide in silicon nitride.
2. The Prior Art
It has been recently discovered that the crystal lattice of silicon nitride can accommodate other atoms, both metallic and non-metallic, in fairly large amounts. For example, recent work has shown that aluminum and oxygen can enter the silicon nitride lattice to give, upon sintering, what is believed to be a solid solution of aluminum oxide in the silicon nitride matrix. This material is commonly referred to as "SIALON". The letters which make up the term "SIALON" are the letters taken from the chemical abbreviation for the elements therein, that is, silicon, aluminum, oxygen and nitrogen. Recent publications which discuss some of the properties of SIALON are to be found in Nature Physical Science, K. H. Jack and W. I. Wilson, Vol. 238, p. 28, July 10, 1972; and Nature, Vol. 238, p. 128, July 21, 1972.
Importantly, it has been discovered that, although the coefficient of thermal expansion is very low for silicon nitride, the inclusion of certain quantities of aluminum oxide within the crystal lattice provides a refractory material with an even lower coefficient of thermal expansion. In addition to the desirable feature of a low coefficient of thermal expansion, SIALON is also known for its compatibility with molten metals, hardness, generally low coefficient of friction, and chemical stability including resistance to oxidation at temperatures up to at least 1200.degree.C.
In addition to the foregoing desirable features as a ceramic material, SIALON also behaves as a ceramic and lends itself well to certain well-known fabrication techniques including, for example, slip casting, extrusion, and pressing to produce a prefired shape.
Accordingly, in view of the desirable characteristics of SIALON coupled with the relative ease of fabrication, an entirely new dimension has been opened for the use of SIALON in numerous high temperature and refractory environments including, for example, molten metal handling components such as continuous casting snorkles, sliding gates, nozzles and furnace and kiln furniture. Other hostile environment applications suitable for a refractory material such as SIALON include catalyst supports, seals on rotary engines, high temperature components in gas turbines, spark plug insulators, abrasives, and cutting tool inserts.
It is readily apparent, therefore, that with the recent discovery of the many desirable properties of SIALON a greatly increased demand will be placed for the production of this material. However, since silicon nitride is not found in nature and is relatively expensive to produce, there exists a need to produce SIALON from readily available raw material sources. An even further advancement would be to provide a process whereby the final product (an intimate dispersion of aluminum oxide and silicon nitride) is readily produced from the initial reactants without further grinding, mixing or otherwise mulling the products of the reaction. An even further advancement would be to provide a process for producing SIALON from raw materials which are conventionally considered to be solid waste products.
Such developments and features are embodied in the present invention.